The plots of  $\frac{1}{{\mathrm{X}}_{\mathrm{A}}}VS.\frac{1}{Y_A}\left(Where X_A\& Y_{A}are the mole fraction of liquid A in liquid and vapour phase respectively\right)$is linear with slope and intercepts respectively

    ${\mathrm{Y}}_{\mathrm{A}}=\frac{{\mathrm{P}}_{\mathrm{A}}}{{\mathrm{P}}_{\mathrm{T}}}=\frac{{\mathrm{P}}_{\mathrm{A}}}{{\mathrm{P}}_{\mathrm{A}}+{\mathrm{P}}_{\mathrm{B}}}$

$=\frac{{\mathrm{x}}_{\mathrm{A}}{\mathrm{p}}_{\mathrm{A}}^{\mathrm{O}}}{{\mathrm{X}}_{\mathrm{A}}{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}\times {\mathrm{X}}_{\mathrm{B}}{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}}$

$\frac{1}{{\mathrm{Y}}_{\mathrm{A}}}=\frac{X_A{P}_A^O+X_B{P}_B^o}{X_A{P}_A^O}$

$=1+\frac{{\mathrm{Y}}_{\mathrm{B}}{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}}{{\mathrm{X}}_{\mathrm{A}}{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}$

$\frac{1}{{\mathrm{Y}}_{\mathrm{A}}}-1=\frac{(1-X_A)P_B^O}{X_A{P}_A^O}$

$=\frac{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}-{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}{\mathrm{X}}_{\mathrm{A}}}{{\mathrm{X}}_{\mathrm{A}}{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}=\frac{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}}{{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}\times \frac{1}{{\mathrm{X}}_{\mathrm{A}}}-\frac{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}}{{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}$

$\frac{1}{{\mathrm{Y}}_{\mathrm{A}}}-1+\frac{P_B^O}{P_A^O}=\left(\frac{P_B^O}{P_A^O}\right)\frac{1}{X_A}$

mx +C=y

$\therefore \mathrm{m}=\frac{{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}},\mathrm{C}=\frac{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}-{\mathrm{P}}_{\mathrm{A}}^{\mathrm{O}}}{{\mathrm{P}}_{\mathrm{B}}^{\mathrm{O}}}$